In large bowl, cream butter and brown sugar, then add mashed banana and beat thoroughly. Add yogurt and vanilla, and beat well.

In a separate bowl, mix together flour, baking soda, cinnamon, salt and nuts, and quickly combine with banana mixture. Fold in 1/3 of the beaten egg whites, then the rest of the whites, until no streaks of white remain. Spread evenly in prepared pan.

Bake for about 30-35 minutes. When done, the top will spring back when pressed lightly with a finger. Cool, then sprinkle with powdered sugar. Cut into bars. Cut the long side into 5 pieces and the short side into 8 pieces.